1. Field of the Invention
The present invention relates to probe cards and more particularly, to a high-frequency probe card that requires less installation space for transmission lines.
2. Description of the Related Art
A probe card is a testing card having a circuit board and a plurality of downwardly extending probes for probing the contact pads on an integrated semiconductor wafer for transmitting test signals subject to the control of a software from a test machine to the integrated circuit of the wafer, thereby performing the wafer level test automatically. The density of the arrangement of the probes must be corresponding to the pitch of the contact pads on the wafer. Further, in order to fit various wafers manufactured by various integrated circuit technologies, the design of the circuit board of a probe card may not use a specific wiring layout for signal transmission between the test machine and the probes. Alternatively, a jumping connection structure of transmission lines may be used and connected between the lead wires probed by the test machine and the probes for allowing signal transmission from the test machines to the probes.
Comparing to the probes of a probe card, the diameter of the transmission line is much greater than the diameter of the probe. When connecting the transmission lines to the probes respectively, the transmission lines become densely arranged in the area adjacent to the probes. Following the sophisticated and versatile circuit design of semiconductor wafer, probe cards must be made having a high count of probes to meet wafer level test requirements of multiple test-items for testing circuit device characteristics quickly. In consequence, the installation density of the jumping connection structure of transmission lines becomes critical. In some cases, for example, in the cantilever-type probe card shown in FIG. 12, transmission lines are stacked and crossed over one another for positive connection to their respective probes. This design complicates the module engineering of jumping transmission lines and increases its difficulty level. As considering a coaxial cable for the transmission of high-frequency signals, an electrically insulating plastic layer having a predetermined wall thickness surrounds an axial wire and also be surrounded by an electrically grounded metal shield so that the characteristic impedance of the transmitted signal propagating by the axial wire can be maintained. However, in order to prevent the transmitted signal from characteristic impedance mismatching that may be resulted from dielectric loss of a parasitic capacitance induced by the plastic layer, the plastic layer must have a certain wall thickness determined subject to its dielectric constant. No matter what kinds of the plastic layer of a coaxial cable used, the entire diameter of the coaxial cable is much greater than its axial wire, thus requiring more installation space of the transmission lines. Therefore, a conventional high-frequency probe card does not allow the dense arrangement of the probes respectively connecting to the coaxial cables for high-frequency test probing.
A probe card may install multi-layer circuit structure of the so-called space converter. For example, FIG. 13 shows a conventional vertical-type probe card 5. According to this design, the vertical-type probe card 5 comprises a circuit board 5a, a space converter 5b, and a plurality of probes 5c stacked at different elevations. The circuit board 5a has arranged thereon a predetermined circuit layout extending from the top surface to the bottom surface for signal transmission. The circuit at the top surface of the circuit board 5a is adapted for the contact of the probing pins 6 of the test machine. The circuit at the bottom surface of the circuit board 5a is for the connection of the space converter 5b. The space converter 5b is formed of a MLC (Multi-Layer Ceramic) or MLO (Multi-Layer Organic) structure. A plurality of electric contacts are arranged on the top and bottom sides of the space converter 5b with different pitches for the respectively electrical connection to the circuit board 5a and the probes 5c. The space converter 5b has laminated circuit layout therein formed by means of the application of semiconductor manufacturing. The pitch of the circuit layout of the space converter 5b that is closer to the probes 5c is relatively smaller so that the lead wires of the circuit board 5a can be respectively conducted to the densely arranged probes 5c, achieving the space conversion effect between the circuit board 5a and the probes 5c to probe the densely arranged electronic devices on the wafer 7. However, the fabrication of the space converter is subject to the application of a micro electromechanical process or thin-film manufacturing, and a specific insulating material like a ceramic substrate may be needed for the base, thereby resulting in a high manufacturing cost greater than the fabrication of the circuit board. There is a demand for a high-frequency probe card that uses an economic circuit structure for signal transmission, maintains characteristic impedance of high-frequency signaling and provides high reliability testing.